logging: change format from thread_local to static
See discussion at https://gitlab.desy.de/constellation/constellation/-/merge_requests/288#note_1034548:
External loggers might use a different thread, so thread_local
does not work.
I checked and it seems that the log format and the log section is not used anywhere in the framework (at least I couldn't find a reference to it). The reporting level is used, but only on a global level, so static should be fine.